Forest fragmentation increases the amount of edges in the landscape. Differences in wind, radiation and vegetation structure create edge-to-interior gradients in forest microclimate, and these gradients are likely to be more pronounced during droughts and heatwaves. Although the effects of climate extremes on edge influences have potentially strong and long-lasting impacts on forest understory biodiversity, they are not well understood and are not often considered in management and landscape planning.

Here we used a novel method of retrospectively quantifying growth to assess biologically relevant edge influences likely caused by microclimate using Hylocomium splendens, a moss with annual segments. We examined how spatio-temporal variation in drought across three years and 46 sites in central Sweden, affected the depth and magnitude of edge influences. We also investigated whether edge effects during drought are influenced by differences in forest structure.

Edge effects were almost twice as strong in the drought year as in the non-drought years, but we did not find clear evidence that they penetrated deeper into the forest in the drought year. Edge influences were also greater in areas that had fewer days with rain during the drought year. Higher levels of forest canopy cover and tree height buffered the magnitude of edge influence in times of drought.

Our results demonstrate that edge effects are amplified by drought, suggesting that fragmentation effects are aggravated when droughts become more frequent and severe. Our results suggest that dense edges and buffer zones with high canopy cover can be important ways to mitigate negative drought impacts in forest edges.

The data is from 46 boreal forest edges in central Sweden. The datasets consist of measurements of yearly growth of the moss Hylocomium splendens along edge-to-interior transects, as well as data on site-level, including canopy cover, tree height and drought intensity.

Moss measurements: At each forest edge, we collected shoots of the moss H. splendens along edge-to-interior transects at the following distances: 0, 5, 10, 20, 30 and 50 m. We collected three moss samples (minimum 7 shoots) perpendicular to the transect at each distance. We measured yearly growth three years back in time (2016, 2017 and 2018, one segment per year) of 21 moss-shoots at each distance, resulting in a total of 17 293 measurements. A more extensive explanation of these measurements is found in the method section of the article.

Site-level data: Forest structural variables were measured at five locations in the forest interior. These five locations were approximately 10 m apart along a transect parallel to the edge. Canopy cover was calculated based on hemispherical photos, using the software ImageJ v. 1.53A with the plug-in Hemisperical 2.0. Tree height was measured with a clinometer and measure-tape.

Spatial variation in drought intensity was calculated using a novel high temporal-spatial resolution dataset called HIPRAD (HIgh-resolution Precipitation from gauge-adjusted weather RADar). We calculated two drought indices: First, we summed the total precipitation during the period 1st May–31st August 2018. Second, we counted the number of days with rainfall over 3 mm during this period.

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